This invention relates to an apparatus and a method for suppressing a fire. More particularly, a gas generator produces an elevated temperature first gas which interacts with a vaporizable liquid to generate a second gas having flame suppressing capabilities.
Fire involves a chemical reaction between oxygen and a fuel which is raised to its ignition temperature by heat. Fire suppression systems operate by any one or a combination of the following: (i) removing oxygen, (ii) reducing the system temperature, (iii) separating the fuel from oxygen, and (iv) interrupting the chemical reactions of combustion. Typical fire suppression agents include water, carbon dioxide, dry chemicals, and the group of halocarbons collectively known as Halons.
The vaporization of water to steam removes heat from the fire. Water is an electrical conductor and its use around electrical devices is hazardous. However, in non-electrical situations, when provided as a fine mist over a large area, water is an effective, environmentally friendly, fire suppression agent.
Carbon dioxide (CO.sub.2) gas suppresses a fire by a combination of the displacement of oxygen and absorption of heat. Carbon dioxide gas does not conduct electricity and may safely be used around electrical devices. The carbon dioxide can be stored as compressed gas, but requires high pressure cylinders for room temperature storage. The cylinders are heavy and the volume of compressed gas limited. Larger quantities of carbon dioxide are stored more economically as a liquid which vaporizes when exposed to room temperature and atmospheric pressure.
When exposed to room temperature and atmospheric pressure, the expansion characteristics of liquid CO.sub.2 are such that approximately one third of the vessel charge freezes during the blow down process. Only about two thirds of the CO.sub.2 is exhausted in a reasonable time. The remainder forms a dry ice mass which remains in the storage vessel. While the dry ice eventually sublimes and exits the vessel, the sublimation period is measured in hours and is of little use in fire suppression.
The problem with liquid carbon dioxide based fire suppression systems is worse when low temperature operation is required. At -65.degree. F., the vapor pressure of carbon dioxide is about 0.48 MPa (70 psig) (compared to 4.8 MPa (700 psig) at 70.degree. F.) which is totally inadequate for rapid expulsion. The vessel freeze-up problem is worse. About 50% of the liquid carbon dioxide solidifies when exposed to -65.degree. F. and atmospheric pressure.
Improved carbon dioxide suppression systems add pressurized nitrogen to facilitate the rapid expulsion of carbon dioxide gas at room temperature. The pressurized nitrogen does not resolve the freezing problem at low temperatures and at upper service extremes, about 160.degree. F., the storage pressure is extremely high, dictating the use of thick, heavy, walled storage vessels.
Chemical systems extinguish a fire by separating the fuel from oxygen. Typical dry chemical systems include sodium bicarbonate, potassium bicarbonate, ammonium phosphate, and potassium chloride. Granular graphite with organic phosphate added to improve effectiveness, known as G-1 powder, is widely used on metal fires. Other suitable dry compounds include sodium chloride with tri-calcium phosphate added to improve flow and metal stearates for water repellency, dry sand, talc, asbestos powder, powdered limestone, graphite powder, and sodium carbonate. Dry chemical systems are delivered to a fire combined with a pressurized inert gas or manually such as with a shovel. The distribution system is inefficient for large fires and a significant amount of time is required to deliver an effective quantity of the dry powder to suppress a large fire.
The most efficient fire suppression agents are Halons. Halons are a class of brominated fluorocarbons and are derived from saturated hydrocarbons, such as methane or ethane, with their hydrogen atoms replaced with atoms of the halogen elements bromine, chlorine, and/or fluorine. This substitution changes the molecule from a flammable substance to a fire extinguishing agent. Fluorine increases inertness and stability, while bromine increases fire extinguishing effectiveness. The most widely used Halon is Halon 1301, CF.sub.3 Br, trifluorobromomethane. Halon 1301 extinguishes a fire in concentrations far below the concentrations required for carbon dioxide or nitrogen gas. Typically, a Halon 1301 concentration above about 3.3% by volume will extinguish a fire.
Halon fire suppression occurs through a combination of effects, including decreasing the available oxygen, isolation of fuel from atmospheric oxygen, cooling, and chemical interruption of the combustion reactions. The superior fire suppression efficiency of Halon 1301 is due to its ability to terminate the runaway reaction associated with combustion. The termination step is catalytic for Halon 1301 due to the stability of bromine radicals (Br.circle-solid.) formed when Halon 1301 is disposed on a combustion source.
When unreacted Halon 1301 migrates into the stratosphere, sunlight breaks down the Halon 1301 forming bromine radicals. Br.circle-solid. then reacts to consume ozone in an irreversible manner. EQU Br.circle-solid.+O.sub.3 .fwdarw.BrO.circle-solid.+O.sub.2
In view of the current recognition that ozone depletion is a serious environmental problem, a move is on to identify: (i) fire suppression agents having a less severe environmental impact than Halon and (ii) devices to deliver these more environmentally friendly agents.